I believe most of it is to do with zbuffer.If i make it 400x400, as i explained, I get a perfect render,but I drop from 120FPS to 43FPS, which I fear may not be an acceptablespeed, considering I am measuring this without drawing any maps, etc.

I'm still not sure what the problem actually is. Maybe you can show two different images, one with a fine render and one with a bad one. I can't see anything bad on the one that you've posted without any comparision.